This application is a 371 of PCT/EP98/06414 filed Oct. 9, 1998.
The present invention relates to a process for the selective preparation of acetic acid by catalytic gas-phase oxidation of ethane and/or ethylene in the presence of a palladium-containing catalyst.
The oxidative dehydrogenation of ethane to ethylene in the gas phase at temperatures of  greater than 500xc2x0 C. is known, for example, from U.S. Pat. No. 4,250,346, U.S. Pat. No. 4,524,236 and U.S. Pat. No. 4,568,790.
Thus, U.S. Pat. No. 4,250,346 describes the use of a catalyst composition comprising the elements molybdenum, X and Y in a ratio of a:b:c for converting ethane into ethylene, where X is Cr, Mn, Nb, Ta, Ti, V and/or W and Y is Bi, Ce, Co, Cu, Fe, K, Mg, Ni, P, Pb, Sb, Si, Sn, TI and/or U and a is 1, b is from 0.05 to 1 and c is from 0 to 2. The total value of c for Co, Ni and/or Fe has to be less than 0.5. The reaction is preferably carried out in the presence of added water. The disclosed catalysts can likewise be used for the oxidation of ethane to acetic acid, with the efficiency of the conversion to acetic acid being about 18% at an ethane conversion of 7.5%.
The abovementioned documents are concerned mainly with the preparation of ethylene, less with the targeted preparation of acetic acid.
In contrast, EP-B-0 294 845 describes a process for the selective preparation of acetic acid from ethane, ethylene or mixtures thereof using oxygen in the presence of a catalyst mixture comprising at least A.) a calcined catalyst of the formula MoxVy or MoxVyZy, where Z may be one or more of the metals Li, Na, Be, Mg, Ca, Sr, Ba, Zn, Cd, Hg, Sc, Y, La, Ce, Al, TI, Ti, Zr, Hf, Pb, Nb, Ta, As, Sb, Bi, Cr, W, U, Te, Fe, Co and Ni and x is from 0.5 to 0.9, y is from 0.1 to 0.4 and z is from 0.001 to 1, and B.) an ethylene hydration catalyst and/or ethylene oxidation catalyst. The second catalyst component B is, in particular, a molecular sieve catalyst or a palladium-containing oxidation catalyst. When using the catalyst mixture described and passing a gas mixture comprising ethane, oxygen, nitrogen and water vapor through the reactor containing the catalyst, the maximum selectivity is 27% at an ethane conversion of 7%. According to EP 0 294 845, the high conversions of ethane are achieved only using the catalyst mixture described, but not using a single catalyst comprising the components A and B.
A further process for preparing a product comprising ethylene and/or acetic acid is described in EP-B-0 407 091. Here, ethane and/or ethylene and a gas comprising molecular oxygen is brought into contact with a catalyst composition comprising the elements A, X and Y at elevated temperature. A is ModReeWf, X is Cr, Mn, Nb, Ta, Ti, V and/or W and Y is Bi, Ce, Co, Cu, Fe, K, Mg, Ni, P, Pb, Sb, Si, Sn, TI and/or U. The maximum selectivities which can be achieved when using the catalyst described in the oxidation of ethane to acetic acid are 78%. Further by-products formed are carbon dioxide, carbon monoxide and ethylene.
The German Patent Application P 19630832.1, which is not a prior publication, describes a process for the selective preparation of acetic acid from a gaseous feed comprising ethane, ethylene or mixtures thereof plus oxygen at elevated temperature. The feed is brought into contact with a catalyst comprising the elements Mo, Pd, X and Y in combination with oxygen.
Here, X is one or more elements selected from the group consisting of Cr, Mn, Nb, Ta, Ti, V, Te and W and Y is one or more elements selected from the group consisting of B, Al, Ga, In Pt, Zn, Cd, Bi, Ce, Co, Rh, Ir, Cu, Ag, Au, Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, TI and U.
The gram atom ratios of the corresponding elements are given as follows:
a(Mo)=1; b(Pd) greater than 0; c(X) greater than 0; and d(Y)=0-2.
The catalysts described in the abovementioned application give a maximum space-time yield of 149 kg/hm3 at an acetic acid selectivity of  greater than 60 mol %. Space-time yields characterize the amount of acetic acid produced per unit time and catalyst volume. Higher space-time yields are desirable since the size of the reactors and the amount of circulated gas can be reduced thereby.
It is therefore an object of the invention to provide a process which allows ethane and/or ethylene to be oxidized to acetic acid in a simple and targeted manner and at high selectivity and space-time yield under reaction conditions which are as mild as possible.
It has surprisingly been found that it is possible to oxidize ethane and/or ethylene to acetic acid under relatively mild conditions in a simple manner at high selectivity and excellent space-time yields when using a catalyst comprising the elements molybdenum and palladium and one or more elements selected from the group consisting of chromium, manganese, niobium, tantalum, titanium, vanadium, tellurium and/or tungsten.
The present invention accordingly provides a process for the selective preparation of acetic acid from a gaseous feed comprising ethane, ethylene or mixtures thereof plus oxygen at elevated temperature, wherein the gaseous feed is brought into contact with a catalyst comprising the elements Mo, Pd, X and Y in the gram atom ratios a:b:c:d in combination with oxygen
MOaPdbXcYdxe2x80x83xe2x80x83(I)
where the symbols X and Y have the following meanings:
X is one or more elements selected from the group consisting of: Cr, Mn, Ta, Ti, V, Te and W, in particular V and W;
Y is one or more elements selected from the group consisting of: B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co, Cu, Rh, Ir, Au, Ag, Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Nb, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, TI and U, in particular Nb, Ca, Sb and Li.
The indices a, b, c and d are the gram atom ratios of the corresponding elements, where
a=1,
b=0.0001-0.01,
c=0.4-1 and
d=0.005-1.
If X and Y represent a plurality of different elements, the indices c and d can likewise assume a plurality of different values.
Furthermore, the present invention provides a catalyst for the selective preparation of acetic acid comprising the elements Mo, Pd, X and Y in the gram atom ratios a:b:c:d in combination with oxygen.
The gram atom ratios a:b:c:d are preferably within the following ranges:
a=1;
b=0.0001-0.005;
c=0.5-0.8 and
d=0.01-0.3.
Palladium contents in the catalyst which are above the upper limit indicated promote carbon dioxide formation in the process of the invention. Furthermore, higher palladium contents are generally also avoided because they make the catalyst unnecessarily expensive. On the other hand, palladium contents below the limit indicated favor formation of ethylene.
Apart from the elements molybdenum and palladium, the catalyst used according to the invention preferably further comprises vanadium, niobium, antimony and calcium in combination with oxygen. The gram atom ratios a:b:c1:d1:d2:d3 of the elements Mo:Pd:V:Nb:Sb:Ca are preferably as follows:
a (Mo)=1;
b (Pd)=0.0001-0.005, in particular 0.0001-0.001;
c1 (V)=0.4-1.0;
d1 (Nb)=0.01-0.2;
d2 (Sb)=0.01-0.3;
d3 (Ca)=0.01-0.3.
Examples of such catalyst compositions which are preferably used in the process of the invention are:
Mo1.00Pd0.0005V0.45Nb0.03Sb0.01Ca0.01 
Mo1.00Pd0.0005V0.45Nb0.03Sb0.01Ca0.01K0.015 
Mo1.00Pd0.00075V0.45Nb0.03Sb0.01Ca0.01 
Mo1.00Pd0.00075V0.55Nb0.03Sb0.01Ca0.01 
Mo1.00Pd0.00075V0.45Nb0.06Sb0.01Ca0.01 
Mo1.00Pd0.0008V0.55Nb0.06Sb0.01Ca0.01 
Mo1.00Pd0.00085V0.55Nb0.06Sb0.01Ca0.01 
Mo1.00Pd0.00075V0.55Nb0.09Sb0.01Ca0.01 
Mo1.00Pd0.0008V0.50Nb0.15Te0.01Ca0.01 
Mo1.00Pd0.00075V0.50Nb0.09W0.01Pd0.0003 
The catalysts used according to the invention can be prepared by conventional methods. These start from a slurry, in particular an aqueous solution, comprising the individual starting components of the elements in the appropriate proportions.
The starting materials of the individual components for preparing the catalyst of the invention are, apart from the oxides, preferably water-soluble substances such as ammonium salts, nitrates, sulfates, halides, hydroxides and salts of organic acids which can be converted into the corresponding oxides by heating. To mix the components, aqueous solutions or suspensions of the metal salts are prepared and mixed.
In the case of molybdenum, it is advisable to use the corresponding molybdates, e.g. ammonium molybdate, as starting compounds because of their commercial availability.
Suitable palladium compounds are, for example, palladium(II) chloride, palladium(II) sulfate, tetramminepalladium(II) nitrate, palladium(II) nitrate and palladium(II) acetylacetonate.
The reaction mixture obtained is then stirred at from 50 to 100xc2x0 C. for from 5 minutes to 5 hours. The water is subsequently removed and the catalyst which remains is dried at a temperature of from 50 to 150xc2x0 C., in particular from 80 to 120xc2x0 C.
If the resulting catalyst is subsequently subjected to a further calcination process, it is advisable to calcine the dried and pulverized catalyst at a temperature in the range from 100 to 800xc2x0 C., in particular from 200 to 500xc2x0 C., in the presence of nitrogen, oxygen or an oxygen-containing gas. The time is from 2 to 24 hours.
The catalyst can be used without a support material or can be mixed with or applied to an appropriate support material. Suitable support materials are customary support materials such as porous silicon dioxide, ignited silicon dioxide, kieselguhr, silica gel, porous or nonporous aluminum oxide, titanium dioxide, zirconium dioxide, thorium dioxide, lanthanum oxide, magnesium oxide, calcium oxide, barium oxide, tin oxide, cerium dioxide, zinc oxide, boron oxide, boron nitride, boron carbide, boron phosphate, zirconium phosphate, aluminum silicate, silicon nitride or silicon carbide or else glass, carbon fiber, metal oxide or metal meshes or corresponding monoliths.
Preferred support materials have a surface area of less than 100 m2/g. Preferred support materials are silicon dioxide and aluminum oxide having a low specific surface area. The catalyst can be used as a heterogeneous oxidation catalyst after shaping as regularly or irregularly shaped support bodies or else in powder form.
The reaction can be carried out in a fluidized bed or in a fixed-bed reactor. For use in a fluidized bed, the catalyst is milled to a particle size in the ranqe from 10 to 200 xcexcm.
The gaseous feed comprises ethane and/or ethylene which are fed to the reactor as pure gases or in admixture with one or more other gases. Examples of such additional or carrier gases are nitrogen, methane, carbon monoxide, carbon dioxide, air and/or water vapor. The gas comprising molecular oxygen can be air or a gas comprising more or less molecular oxygen than air, e.g. oxygen. The proportion of water vapor can be in the range from 0 to 50% by volume. Higher water vapor concentrations would make the work-up of the aqueous acetic acid formed unnecessarily expensive for process reasons. The ratio of ethane/ethylene to oxygen is advantageously in the range from 1:1 to 10:1, preferably from 2:1 to 8:1. Relatively high oxygen contents are preferred since the achievable ethane conversion and thus the yield of acetic acid is higher. Oxygen or the gas comprising molecular oxygen is preferably added in a concentration range outside the explosive limtis under the reaction conditions since this makes the process easier to carry out. However, it is also possible to employ an ethane/ethylene to oxygen ratio within the explosive limits.
The reaction is carried out at temperatures of from 200 to 500xc2x0 C., preferably from 200 to 400xc2x0 C. The pressure can be atmospheric or superatmospheric, e.g. in the range from 1 to 50 bar, preferably from 1 to 30 bar.
The reaction can be carried out in a fixed-bed or fluidized-bed reactor.
Advantageously, ethane is first mixed with the inert gases such as nitrogen or water vapor before oxygen or the gas comprising molecular oxygen is fed in. The mixed gases are preferably preheated to the reaction temperature in a preheating zone before the gas mixture is brought into contact with the catalyst. Acetic acid is separated from the gas leaving the reactor by condensation. The remaining gases are recirculated to the reactor inlet where oxygen or the gas comprising molecular oxygen and also ethane and/or ethylene are metered in.
On comparing the catalysts of the invention with those of the prior art, it is found that the present catalysts achieve higher space-time yields and acetic acid selectivities under identical reaction conditions (reaction feed gas, pressure, temperature).
When using the catalyst of the invention, the selectivity in the oxidation of ethane and/or ethylene to acetic acid isxe2x89xa770 mol %, preferablyxe2x89xa780 mol %, in particularxe2x89xa790 mol %, and the space-time yield is greater than 150 kg/hm3, in particular greater than 200 kg/hm3, preferably greater than 300 kg/hm3, so that the process of the invention enables, in comparison with the prior art, an increase in the acetic acid yields to be achieved in a simple manner while simultaneously reducing the formation of undesired by-products.